1. Field of the Invention
The present invention is related to the field of oil and gas wellbore logging. More specifically, the present invention is related to a system for determining the fractional gas saturation of an earth formation from within a cased wellbore which may be at least partially filled with gas.
2. Discussion of the Related Art
Oil and gas wellbores are typically surveyed, or logged, while they are still uncased, or directly exposed to the earth formations through which they are drilled. Logging is performed in order to determine certain properties of the earth formations. An instrument known in the art which is used to survey open wellbores is called a neutron logging tool. The neutron logging tool typically includes a source of high energy, or fast, neutrons, and at least one detector which is sensitive to lower energy neutrons. Neutron logging tools known in the art are primarily responsive to the concentration of hydrogen nuclei in the earth formation. Hydrogen nuclei are typically present in high concentrations in fluids which can fill void spaces, or porosity, which can exist in some earth formations. Oil and gas, if present in a particular earth formation, are generally present in the porosity. Neutron tools provide measurements which can be used in determining the fractional volume of the earth formation which is occupied by the porosity, and therefore provide an indication of the volume of oil and gas which may be present in the formation.
Measurements made by neutron logging tools known in the art are highly sensitive to the presence of gas. Even though gas consists essentially of hydrogen and carbon, it has a much lower density than water or oil. Therefore gas typically provides lower overall hydrogen nucleus concentration in a particular earth formation than does water or oil. Consequently, neutron tools are useful for determining the depth in the wellbore at which, for example, gas is in contact with oil or water within the porosity of a particular formation. Neutron logs are also useful for determining changes in the depth of the gas/oil or gas/water contact in the formation which can occur over a period of time as fluids are extracted from the formation.
A drawback to using the neutron tool known in the art for determining gas contacts and gas saturation is that wellbores typically are "completed" in order to extract fluids from the formation, and gas can be present within the completed wellbore during extraction of fluids from the formation, as will be further explained.
Completing a wellbore typically includes the step of cementing a steel pipe, or casing, to a depth within the wellbore which at least includes the formation from which fluids are to be extracted. The cement filling the annular volume external to the casing and internal to the wellbore typically has a high hydrogen content, which can affect the measurements of the neutron tool known in the art.
During the production of fluids from the formation, gas may enter the wellbore from the formation either as gas exsolved from oil in the formation, or as gas produced directly from the formation. The neutron tools known in the art are typically calibrated to measure water-filled porosity of the formation from within a wellbore which is liquid-filled. Gas in the wellbore can affect the measurements of the neutron tool known in the art. Since the fractional volume of the wellbore which may be filled with gas is difficult to determine, it is impractical to adjust the measurements of the neutron tool known in the art for the amount of gas in the wellbore.
It is an object of the present invention to provide a neutron tool which can measure gas saturation in the formation in a wellbore having a cemented casing.
It is a further object of the present invention to provide a neutron tool which can measure gas saturation in the formation from within a wellbore which is at least partially filled with gas.